A device for regulating the volume rate of delivery of flow from a positive displacement pump is known from Swiss Pat. No. 594,137 and U.S. Pat. No. 4,164,240. In the device described there, the rotary shutter has a pair of raised ribs or cam lobes which alternately open and close two oppositely disposed nozzles. When opened, the two nozzles permit communication with a zero-pressure reservoir, and thus temporarily interrupt the supply of fluid to the output side. When the return flow nozzles are closed, however, the fluid in the pump is forced to flow through a single non-return or check valve to the output connection of the device. The edges of the rotary shutter are angled so as to permit material removal of the terminal of the return flow nozzles so as to generate during the initial operation an optimum gap as between the rotary shutter and the return flow nozzles.
When the openings of the return flow nozzles in the known device are round, which is preferable for reasons of simplified manufacture, the sound level which is generated by the device in operation is no less than 80 dB (A). This sound level is produced by the transformation of pressure peaks in the lines and the conduits of the pump into an acoustical signal.